An electret is a material having thereinside a permanent electric charge imparted by injecting charges into an insulating polymer material. Such an electret formed into fibers is widely used for a dust-collecting filter etc.
It is known that a foamed sheet made of a synthetic resin exhibits very high piezoelectric properties comparable to those of ceramics when bubble membranes forming the bubbles and their vicinities are electrified. It has been proposed to apply an electret that uses such a synthetic resin-made foamed sheet to acoustic pickups, various pressure sensors, etc. with high sensitivity of the electret being utilized.
One method used to improve the piezoelectric properties of such a synthetic resin-made foamed sheet is to reduce the size of the bubbles. Patent Literature 1 discloses a method of reducing the size of bubbles. Although higher performance than that of conventional electrets is obtained, this method has a problem in that a complicated process must be used.